Displacement measuring interferometers monitor changes in the position of a measurement object relative to a reference object based on an optical interference signal. The interferometer generates the optical interference signal by overlapping and interfering a measurement beam reflected from the measurement object with a reference beam reflected from the reference object.
In many applications, the measurement and reference beams have orthogonal polarizations and different frequencies. The different frequencies can be produced, for example, by laser Zeeman splitting, by acousto-optical modulation, or internal to the laser using birefringent elements or the like. The orthogonal polarizations allow a polarizing beam splitter to direct the measurement and reference beams to the measurement and reference objects, respectively, and combine the reflected measurement and reference beams to form overlapping exit measurement and reference beams. The overlapping exit beams form an output beam that subsequently passes through a polarizer.
The polarizer mixes polarizations of the exit measurement and reference beams to form a mixed beam. Components of the exit measurement and reference beams in the mixed beam interfere with one another so that the intensity of the mixed beam varies with the relative phase of the exit measurement and reference beams. A detector measures the time-dependent intensity of the mixed beam and generates an electrical interference signal proportional to that intensity. Because the measurement and reference beams have different frequencies, the electrical interference signal includes a xe2x80x9cheterodynexe2x80x9d signal having a beat frequency equal to the difference between the frequencies of the exit measurement and reference beams. If the lengths of the measurement and reference paths are changing relative to one another, e.g., by translating a stage that includes the measurement object, the measured beat frequency includes a Doppler shift equal to 2vnp/xcex, where v is the relative speed of the measurement and reference objects, xcex is the wavelength of the measurement and reference beams, n is the refractive index of the medium through which the light beams travel, e.g., air or vacuum, and p is the number of passes to the reference and measurement objects. Changes in the relative position of the measurement object correspond to changes in the phase of the measured interference signal, with a 2xcfx80 phase change substantially equal to a distance change L of xcex/(np), where L is a round-trip distance change, e.g., the change in distance to and from a stage that includes the measurement object.
Unfortunately, this equality is not always exact. In addition, the amplitude of the measured interference signal may be variable. A variable amplitude may subsequently reduce the accuracy of measured phase changes. Many interferometers include non-linearities such as what are known as xe2x80x9ccyclic errors.xe2x80x9d The cyclic errors can be expressed as contributions to the phase and/or the intensity of the measured interference signal and have a sinusoidal dependence on the change in optical path length pnL. In particular, the first harmonic cyclic error in phase has a sinusoidal dependence on (2xcfx80pnL)/xcex and the second harmonic cyclic error in phase has a sinusoidal dependence on 2(2xcfx80pnL)/xcex. Higher harmonic cyclic errors and sub-harmonic cyclic errors can also be present.
There are also xe2x80x9cnon-cyclic non-linearitiesxe2x80x9d such as those caused by a change in lateral displacement (i.e., xe2x80x9cbeam shearxe2x80x9d) between the reference and measurement beam components of an output beam of an interferometer when the wavefronts of the reference and measurement beam components have wavefront errors. This can be explained as follows.
Inhomogeneities in the interferometer optics may cause wavefront errors in the reference and measurement beams. When the reference and measurement beams propagate collinearly with one another through such inhomogeneities, the resulting wavefront errors are identical and their contributions to the interferometric signal cancel each other out. More typically, however, the reference and measurement beam components of the output beam are laterally displaced from one another, i.e., they have a relative beam shear. Such beam shear causes the wavefront errors to contribute an error to the interferometric signal derived from the output beam.
Moreover, in many interferometry systems beam shear changes as the position or angular orientation of the measurement object changes. For example, a change in relative beam shear can be introduced by a change in the angular orientation of a plane mirror measurement object. Accordingly, a change in the angular orientation of the measurement object produces a corresponding error in the interferometric signal.
The effect of the beam shear and wavefront errors will depend upon procedures used to mix components of the output beam with respect to component polarization states and to detect the mixed output beam to generate an electrical interference signal. The mixed output beam may for example be detected by a detector without any focusing of the mixed beam onto the detector, by detecting the mixed output beam as a beam focused onto a detector, or by launching the mixed output beam into a single mode or multi-mode optical fiber and detecting a portion of the mixed output beam that is transmitted by the optical fiber. The effect of the beam shear and wavefront errors will also depend on properties of a beam stop should a beam stop be used in the procedure to detect the mixed output beam. Generally, the errors in the interferometric signal are compounded when an optical fiber is used to transmit the mixed output beam to the detector.
Amplitude variability of the measured interference signal can be the net result of a number of mechanisms. One mechanism is a relative beam shear of the reference and measurement components of the output beam that is for example a consequence of a change in orientation of the measurement object.
In dispersion measuring applications, optical path length measurements are made at multiple wavelengths, e.g., 532 nm and 1064 nm, and are used to measure dispersion of a gas in the measurement path of the distance measuring interferometer. The dispersion measurement can be used in converting the optical path length measured by a distance measuring interferometer into a physical length. Such a conversion can be important since changes in the measured optical path length can be caused by gas turbulence and/or by a change in the average density of the gas in the measurement arm even though the physical distance to the measurement object is unchanged.
The interferometers described above are often crucial components of scanner systems and stepper systems used in lithography to produce integrated circuits on semiconductor wafers. Such lithography systems typically include a translatable stage to support and fix the wafer, focusing optics used to direct a radiation beam onto the wafer, a scanner or stepper system for translating the stage relative to the exposure beam, and one or more interferometers. Each interferometer directs a measurement beam to, and receives a reflected measurement beam from, a plane mirror attached to the stage. Each interferometer interferes its reflected measurement beams with a corresponding reference beam, and collectively the interferometers accurately measure changes in the position of the stage relative to the radiation beam. The interferometers enable the lithography system to precisely control which regions of the wafer are exposed to the radiation beam.
In many lithography systems and other applications, the measurement object includes one or more plane mirrors to reflect the measurement beam from each interferometer. Small changes in the angular orientation of the measurement object, e.g., pitch and yaw of a stage, can alter the direction of each measurement beam reflected from the plane mirrors. If left uncompensated, the altered measurement beams reduce the overlap of the exit measurement and reference beams in each corresponding interferometer. Furthermore, these exit measurement and reference beams will not be propagating parallel to one another nor will their wave fronts be aligned when forming the mixed beam. As a result, the interference between the exit measurement and reference beams will vary across the transverse profile of the mixed beam, thereby corrupting the interference information encoded in the optical intensity measured by the detector.
To address this problem, many conventional interferometers include a retroreflector that redirects the measurement beam back to the plane mirror so that the measurement beam xe2x80x9cdouble passesxe2x80x9d the path between the interferometer and the measurement object. The presence of the retroreflector insures that the direction of the exit measurement is insensitive to changes in the angular orientation of the measurement object. When implemented in a plane mirror interferometer, the configuration results in what is commonly referred to as a high-stability plane mirror interferometer (HSPMI). However, even with the retroreflector, the lateral position of the exit measurement beam remains sensitive to changes in the angular orientation of the measurement object. Furthermore, the path of the measurement beam through optics within the interferometer also remains sensitive to changes in the angular orientation of the measurement object.
In practice, the interferometry systems are used to measure the position of the wafer stage along multiple measurement axes. For example, defining a Cartesian coordinate system in which the wafer stage lies in the x-y plane, measurements are typically made of the x and y positions of the stage as well as the angular orientation of the stage with respect to the z axis, as the wafer stage is translated along the x-y plane. Furthermore, it may be desirable to also monitor tilts of the wafer stage out of the x-y plane. For example, accurate characterization of such tilts may be necessary to calculate Abbe offset errors in the x and y positions. Thus, depending on the desired application, there may be up to five degrees of freedom to be measured. Moreover, in some applications, it is desirable to also monitor the position of the stage with respect to the z-axis, resulting in a sixth degree of freedom.
To measure each degree of freedom, an interferometer is used to monitor distance changes along a corresponding metrology axis. For example, in systems that measure the x and y positions of the stage as well as the angular orientation of the stage with respect to the x, y, and z axes, at least three spatially separated measurement beams reflect from one side of the wafer stage and at least two spatially separated measurement beams reflect from another side of the wafer stage. See, e.g., U.S. Pat. No. 5,801,832 entitled xe2x80x9cMethod of and Device for Repetitively Imaging a Mask Pattern on a Substrate Using Five Measuring Axes,xe2x80x9d the contents of which are incorporated herein by reference. Each measurement beam is recombined with a reference beam to monitor optical path length changes along the corresponding metrology axes. Because the different measurement beams contact the wafer stage at different locations, the angular orientation of the wafer stage can then be derived from appropriate combinations of the optical path length measurements. Accordingly, for each degree of freedom to be monitored, the system includes at least one measurement beam that contacts the wafer stage. Furthermore, as described above, each measurement beam may double-pass the wafer stage to prevent changes in the angular orientation of the wafer stage from corrupting the interferometric signal. The measurement beams may generated from physically separate interferometers or from multi-axes interferometers that generate multiple measurement beams.
The invention features multiple degrees of freedom measuring interferometers, such as ones that measure multiple degrees of freedom of a plane mirror measurement object. For example, the interferometers may be combined into single optical assemblies. Information about changes in position of the measurement object are derived from multiple ouput beams that each include a component that makes one pass to the measurement object along a common measurement beam path. As a result, such interferometer systems can exhibit reduced non-cyclic errors in certain of the measured degrees of freedom that arise from different sources, e.g., relative misalignments of measurement beams with corresponding differing cosine or cosecant correction factors, wave front errors in the presence of beam shears, non-homogeneities in glass in the presence of beam shears, and temperature gradients. Moreover, spatial separations of certain of the measurement beams contacting the plane mirror measurement object need not be determined by the locations of one or more retroreflectors. Also, the interferometer systems generally involve a reduced number of optical elements.
The interferometer systems may be configured so that the measured multiple degrees of freedom are based on combinations of angular and/or linear displacements of the plane mirror measurement object.
The interferometer systems may further be configured such that the directions of input beams to the multiple degree of freedom measuring interferometers are controlled by one or more dynamic beam steering elements to reduce (e.g., substantially eliminate) beam shear in the interferometers and at detectors as the plane mirror measurement object, thereby reducing one source of non-cyclic errors. The interferometer systems may also be configured such that both the directions of input and output beams to and from the multiple degree of freedom measuring interferometers are controlled by one or more beam steering elements to reduce (e.g., substantially eliminate) the beam shear and changes in the angles of incidence of output beams at detectors as the plane mirror measurement object is tilted.
The multi-axis interferometry system can be incorporated into a lithography system for making semiconductor chips. The multi-axis interferometry system can also be incorporated into a beam writing system.
We now summarize different aspects and features of the invention.
In general, in one aspect, the invention features an apparatus including a multi-axis interferometer for measuring a relative position of a reflective measurement object along multiple degrees of freedom, wherein the interferometer is configured to produce multiple output beams each including information about the relative position of the measurement object with respect to a different one of the degrees of freedom. Each output beam includes a beam component that contacts the measurement object at least one time along a common path, and at least one of the beam components further contacts the measurement object at least a second time along a first path different from the common path.
Embodiments of the apparatus may include any of the following features.
The output beam including the beam component that contacts the measurement object along the common path and along the first path may further include a second beam component that does not contact the measurement object.
One of the degrees of freedom may be distance to the measurement object along a first measurement axis. For example, the output beam including information about the distance to the measurement object along the first measurement axis may include the beam component that contacts the measurement object along the common path and along the first path. In such embodiments, the first measurement axis may be defined by the common path and the first path. For example, each point on the first measurement axis may be equidistant to corresponding points on the common path and the first path.
Furthermore, in addition to the distance measurement along the first measurement axis, another of the beam components may further contacts the measurement object at least a second time along a second path different from the common path. The output beam including the beam component that contacts the measurement object along the common path and along the second path may include information about distance to the measurement object along a second measurement axis defined by the common path and the second path, the second measurement axis being different from the first measurement axis. Moreover, a third of the beam components may further contact the measurement object at least a second time along a third path different from the common path. The output beam including the beam component that contacts the measurement object along the common path and the third path may include information about distance to the measurement object along a third measurement axis defined by the common path and the third path, the third measurement axes being different from the first and second measurement axes. For example, the first and second measurement axes may define a first plane, and the second and third measurement axes may define a second plane different from the first plane.
In addition to the distance measurement along the first measurement axis, a second one of the output beams may include information about an angular orientation of the measurement object with respect to a first rotation axis. For example, the second output beam may include the first-mentioned beam component that contacts the measurement object along the common path and another beam component different from the first beam component, wherein the other beam component contacts the measurement object along a second path different from the common path. The first path may different from the second path, or it might be the same as the common path. The first rotation axis may normal to a plane defined by the common path and the second path. Furthermore, a third one of the output beams may include information about an angular orientation of the measurement object with respect to a second rotation axis different from the first rotation axis. The third output beam may include the first-mentioned beam component that contacts the measurement object along the common path and another beam component different from the first beam component, wherein the other beam component in the third output beam contacts the measurement object along a third path different from the common path. For example, the second rotation axis may be normal to the first rotation axis.
The multi-axis interferometer may produces at least four output beams each including information about a different one of the multiple degrees of freedom. Furthermore, the multi-axis interferometer may provides information with respect to at least five degrees of freedom, or even with respect to at least seven degrees of freedom.
The measurement object may include a plane mirror.
The apparatus may further include a light source configured to direct an input beam into the multi-axis interferometer, the input beam including two components having a heterodyne frequency splitting. Each output beam that contacts the measurement object along the common path is derived from one of the components in the input beam, and each output beam further includes a second component derived from the other one of the components in the input beam. For example, the components of the input beam may have orthogonal polarizations.
The apparatus may further include detectors configured to receive the output beams and generate electrical signals indicative of the information about the relative position of the measurement object with respect to the different degrees of freedom. Moreover, the apparatus may further include a polarization analyzer positioned prior to each detector and configured to pass an interfermediate polarization to those of the components in each of the output beams. Also, the apparatus may further include a fiber-optic pick-up for coupling each output beam to a corresponding detector after it passes through the corresponding polarization analyzer.
The interferometer may be configured to direct a first beam derived from an input beam to contact the measurement object along the common path and separate the first beam into multiple sub-beams after it contacts the measurement object, wherein the sub-beams correspond to the beam components in the output beams that contact the measurement object along the common path. Furthermore, the interferometer may be configured to further direct at least one of the sub-beams to contact the measurement object at least a second time along the first path to define the beam component that contacts the measurement object along the common path and along the first path. Moreover, the interferometer may be configured to derive another sub-beam from the input beam and combine the sub-beam that contacts the measurement object at least twice with the other sub-beam to produce a first one of the output beams, wherein the first output beam includes information about distance to the measurement object along a measurement axis defined by the common path and the first path.
Also, the interferometer may be configured to further direct at least another one of the sub-beams to contact the measurement object at least a second time along a second path different from the common path and different from the first path. For example, the interferometer maybe configured to derive another set of sub-beams from the input beam and combine each of the sub-beams that contacts the measurement object at least twice with a corresponding sub-beam from the other set to produce a corresponding one of the output beams.
The interferometer may also be configured to derive another set of sub-beams from the input beam, combine the sub-beam that contacts the measurement object at least twice with one of the sub-beams from the other set to produce one of the output beams, direct another one of the sub-beams from the other set to contact the measurement object along a second path different from the common path, and combine another one of the sub-beams that contact the measurement object along the common path with the sub-beam that contacts the measurement object along the second path to produce another one of the output beams.
In any of the embodiments in which the sub-beams are referred to, the interferometer may further be configured to: i) combine the first beam after it contacts the measurement object with a primary reference beam to define an intermediate beam; ii) separate the intermediate beam into a set of secondary measurement beams and a set of secondary reference beams; iii) direct each of the secondary measurement beams to contact the measurement object; and iv) recombine each secondary measurement beam after it contacts the measurement object with a one of corresponding secondary reference beams to produce a corresponding one of the output beams, wherein each of the sub-beams corresponds to a different one of the secondary measurement and reference beams.
The interferometer may include: a common polarizing beam-splitter positioned to direct a primary measurement beam derived from an incident input beam to contact the measurement object along the common path; and a return beam assembly configured to receive an intermediate beam including the primary measurement beam from the polarizing beam-splitter, separate the intermediate beam into multiple beams, and direct the multiple beams back to the polarizing beam-splitter. The polarizing beam-splitter may be further positioned to direct a primary reference beam derived from the incident input beam to contact a reflective reference object, wherein the primary measurement beam and primary reference beam correspond to orthogonal polarization components of the incident input beam. The polarizing beam-splitter may be further positioned to recombine the primary measurement and reference beams to form the intermediate beam after they contact the measurement and reference objects, respectively.
In addition, the polarizing beam-splitter may be is positioned to: i) separate the multiple beams into a set of secondary measurement beams and a set of secondary reference beams; ii) direct each of the secondary measurement beams to contact the measurement object; iii) direct each of the secondary reference beams to contact the reference object; and iv) recombine each secondary measurement beam with a corresponding one of the secondary reference beams after they contact the measurement and reference objects, respectively, to form a corresponding one of the output beams.
In such embodiments, each secondary measurement beam may contact the measurement object along a path different from the common the path. Furthermore, the interferometer may include the reference object. Alternatively, the reference object may correspond to another measurement object, such as in a differential plane mirror interferometer. In either case, the reference object may include a plane minor.
The interferometer may further includes a measurement quarter-wave retarder positioned between the polarizing beam-splitter and the measurement object and also a reference quarter-wave retarder positioned between the polarizing beam-splitter and the reference object.
The interferometer further may include an input beam optical assembly including a non-polarizing beam-splitter, wherein the input beam optical assembly is configured to separate a progenitor input beam into the first-mentioned input beam and a second input beam propagating parallel to the first input beam and direct the first and second input beams to the polarizing beam-splitter.
The return beam assembly may include at least one set of fold optics and at least one non-polarizing beam-splitter positioned to separate the intermediate beam into the multiple beams. For example, the set of fold optics may include a retroreflector positioned to receive the intermediate beam prior to any of the non-polarizing beam-splitters. The return beam assembly further may include a beam-splitting assembly including the at least one non-polarizing beam-splitter, wherein the beam-splitting assembly receives the intermediate beam from the retroreflector, generates the multiple beams, and directs the multiple beams back to the polarizing beam splitter along directions parallel to that of the intermediate beam. Moreover, the beam-splitting assembly may include multiple non-polarizing beam-splitters. Also, the return beam assembly further may include a retardation plate positioned between the retroreflector and the beam-splitting assembly, wherein the retardation plate is oriented to reduce polarization rotation of the intermediate beam caused by the retroreflector.
In other embodiments, the set of fold optics may include angle-measuring fold optics and distance-measuring fold optics, wherein the angle-measuring fold optics includes a half-wave retarder positioned to rotate the polarization of at least one of the multiple beams. The angle-measuring fold optics may further include a penta-prism and the distance-measuring optics may include a retroreflector. The non-polarizing beam-splitter may be positioned to receive the intermediate beam before or after any of the fold optics.
In general, in another aspect, the invention features a method including interferometrically producing multiple output beams each including information about a relative position of a measurement object with respect to a different degree of freedom. Each output beam includes a beam component that contacts the measurement object at least one time along a common path. Furthermore, at least one of the beam components further contacts the measurement object at least a second time along a first path different from the common path.
Embodiments of the method may further include any of the features corresponding to those described above for the apparatus.
In general, in another aspect, the invention features an apparatus including a multi-axis interferometer for measuring a relative position of a reflective measurement object along multiple degrees of freedom, wherein the interferometer is configured to produce multiple output beams each including information about the relative position of the measurement object with respect to a different one of the degrees of freedom. Each output beam includes a beam component that contacts the measurement object at least one time along a common path. At least one of the output beams includes another beam component different from the first-mentioned beam component. The other beam component contacts the measurement object at least one time along a first path different from the common path.
Embodiments of the second-mentioned apparatus may include any of the following features.
The output beam including the first beam component that contacts the measurement object along the common path and the other beam component that contacts the measurement object along the first path may include information about the angular orientation of the measurement object with respect to a first rotation axis. For example, the first rotation axis may be normal to a plane defined by the common path and the first path. Furthermore, a second one of the output beams may include information about an angular orientation of the measurement object with respect to a second rotation axis different from the first rotation axis, wherein the second output beam includes the first-mentioned beam component that contacts the measurement object along the common path and another beam component different from the first beam component, wherein the other beam component in the second output beam contacts the measurement object along a second path different from the common path. For example, the second rotation axis may be normal to the first rotation axis.
Further embodiments of the second-mentioned apparatus may include any of the additional features described above for the first-mentioned apparatus.
In general, in another aspect, the invention features a method including interferometrically producing multiple output beams each including information about a relative position of a measurement object with respect to a different degree of freedom, wherein each output beam includes a beam component that contacts the measurement object at least one time along a common path. At least one of the output beams includes another beam component different from the first-mentioned beam component. The other beam component contacts the measurement object at least one time along a first path different from the common path.
Embodiments of the second-mentioned method may further include any of the features corresponding to those described above for the first-mentioned and second-mentioned apparatus.
In another aspect, the invention features a lithography system for use in fabricating integrated circuits on a wafer. The lithography system includes: a stage for supporting the wafer; an illumination system for imaging spatially patterned radiation onto the wafer; a positioning system for adjusting the position of the stage relative to the imaged radiation; and any of the interferometric apparatus described above for monitoring the position of the wafer relative to the imaged radiation.
In another aspect, the invention features another lithography system for use in fabricating integrated circuits on a wafer. This lithography system includes: a stage for supporting the wafer; and an illumination system including a radiation source, a mask, a positioning system, a lens assembly, and any of the interferometric apparatus described above. During operation the source directs radiation through the mask to produce spatially patterned radiation, the positioning system adjusts the position of the mask relative to the radiation from the source, the lens assembly images the spatially patterned radiation onto the wafer, and the interferometry system monitors the position of the mask relative to the radiation from the source.
In another aspect, the invention features a beam writing system for use in fabricating a lithography mask. The beam writing system includes: a source providing a write beam to pattern a substrate; a stage supporting the substrate; a beam directing assembly for delivering the write beam to the substrate; a positioning system for positioning the stage and beam directing assembly relative one another; and any of the interferometric apparatus described above for monitoring the position of the stage relative to the beam directing assembly.
In another aspect, the invention features a lithography method for use in fabricating integrated circuits on a wafer. The lithography method includes: supporting the wafer on a moveable stage; imaging spatially patterned radiation onto the wafer; adjusting the position of the stage; and monitoring the position of the stage using any of the interferometric methods described above.
In another aspect, the invention features another lithography method for use in the fabrication of integrated circuits. This lithography method includes: directing input radiation through a mask to produce spatially patterned radiation; positioning the mask relative to the input radiation; monitoring the position of the mask relative to the input radiation using any of the interferometry methods described above; and imaging the spatially patterned radiation onto a wafer.
In another aspect, the invention features a third lithography method for fabricating integrated circuits on a wafer including: positioning a first component of a lithography system relative to a second component of a lithography system to expose the wafer to spatially patterned radiation; and monitoring the position of the first component relative to the second component using any of the interferometric methods described above.
In another aspect, the invention features a method for fabricating integrated circuits, the method including any of the lithography methods described above.
In another aspect, the invention features a method for fabricating integrated circuits, the method including using any of the lithography systems described above.
In another aspect, the invention features a method for fabricating a lithography mask, the method including: directing a write beam to a substrate to pattern the substrate; positioning the substrate relative to the write beam; and monitoring the position of the substrate relative to the write beam using any of the interferometry methods described above.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In case of conflict with publications, patent applications, patents, and other references mentioned incorporated herein by reference, the present specification, including definitions, will control.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.